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Most formats define pages in terms of some fairly simple `well known' colorspaces, like RGB and CMYK. Some formats (notably PDF) are much more powerful, and allow pages to be constructed with a range of non-standard `spot' inks.
When combined with advanced features such as overprinting, care needs to be taken to ensure that the rendering is exactly as expected.
For example, if a PDF page is constructed to render a page using overprinting it only makes strict sense to render this to a CMYK (or a CMYK + Spots) pixmap. With (say) an RGB pixmap, CMYK colors would be mapped down to RGB as they are plotted, losing the information required to correctly overprint later graphical objects.
Nonetheless, while we might want to get a `true' rendition of the page, we might require it ultimately to appear as an RGB pixmap. As such what we really want is to get a `simulation' of how the overprint would work.
One way to work would be to call the draw device and request a CMYK + Spots rendering, and then to require the caller to convert this to their desired target colorspace manually. This is not in keeping with the general desire in MuPDF to encapsulate functionality in a friendly way.
Therefore, the draw device examines the `separations' field of the pixmap that it is called with to decide how to render.
If there is no separations value supplied (i.e. it is NULL), then the draw device assumes that no form of overprint (or overprint simulation) is required.
If there is a separations value, and there is at least one separation that is not entirely disabled, then the draw device will draw internally to a CMYK + Spots pixmap (where the spots are the non-disabled separations from the separations value). This rendering can safely proceed with overprint processing enabled.
At the end of the render, the draw device will convert down from the CMYK + Spots pixmap to the colorspace of the initial pixmap. Any spot colorants present in the initial pixmap will be populated from the rendered one; any spots that aren't will be converted down to process colors.
Thus by creating the initial pixmap passed into the draw device using a separations object with the colorants correctly set to be composite/spots/disabled as required, overprint or overprint simulation can be controlled as required.